The conventional air-fuel ratio sensors for detecting an air-fuel ratio from the gaseous components in an exhaust gas include, for example, "Apparatus for Measuring Partial Pressure of Predetermined Gaseous Components in Monitored Gas Environment" disclosed in Japanese Patent Laid-Open No. 69690/1977 published on June 9, 1977, and "Inflammable Substance Sensor" disclosed in Japanese Patent Laid-Open No. 66292/1978 published on June 13, 1978, in which a constant or variable voltage is applied to an air-fuel ratio detecting cell to determine the air-fuel ratio on the basis of an amperage obtained. These air-fuel ratio sensors depend much upon the temperature, so that they required to be used at a high temperature. The methods of controlling the temperature of an air-fuel ratio sensor include a method of applying a voltage, in which an AC component is superposed on a DC voltage, to a detecting cell, and controlling the temperature on the basis of the temperature dependency information from the AC component, and a method of determining the internal resistance of a detecting cell on the basis of an AC component, and controlling the temperature on the basis of the internal resistance, as disclosed in Japanese Patent Laid-Open No. 187646/1982 entitled "Method of and Apparatus for Measuring Temperature of Threshold Current Sonde or Oxygen Sonde" and published on Nov. 18, 1982, and Japanese Patent Laid-Open No. 192852/1982 entitled "Temperature-controlled Threshold Current Type Oxygen Concentration Detector" and published on Nov. 27, 1982. Japanese patent Laid-Open No. 192849/1982 entitled "Measuring Output Temperature compensated Threshold Current Type Oxygen Concentration Detector" and published on Nov. 27, 1982 discloses a method in which the steps of applying a voltage to a detecting cell to measure a threshold current, and determining the internal resistance are repeatedly carried out. In this method, the signal processing is done by determining a temperature compensation factor, and then carrying out computations.
However, in the conventional air-fuel ratio sensors, in which the detection of an air-fuel ratio and the obtaining of temperature information are carried out on the basis of a DC component and an AC component, respectively, it is necessary that the AC component superposed on the DC component so as to obtain an air-fuel ratio output from the air-fuel ratio sensors be separated therefrom. This makes it necessary to provide a special signal processor or computing element.
Japanese Patent Laid-Open No. 163862/1982 entitled "Detecting Circuit for Oxygen Concentration Sensor" and published on Oct. 8, 1982 discloses a circut structure for outputting a signal representative of the possibility that the detecting cell will be normally operated, on the basis of the internal resistance of a detecting cell, and outputting a signal received from the detecting cell. However, the controlling of the temperature of a detecting cell is not disclosed in the conventional circuit.